You might have hit a cul-de-sac while trying to find more information considering that the error code and error message didn’t result in any relevant information by search engines. So I dug in deep and got the real issue behind it. But not anymore and it's time to break the mystry.

This is due to the reason that ACS rejects token requests when ACS internal resources are temporarily consumed by a high token request rate from all namespaces. In this case, ACS returns an HTTP 503 "Service unavailable" error with the ACS90046 or ACS60021 (ACS Busy) error codes.